1. Statement of the Technical Field
The present invention is directed to the field of directional couplers, and more particularly, to directional couplers having a miniaturized design.
2. Description of the Related Art
Directional couplers are four-port circuits typically used for sampling of the input power for use in signal monitoring circuits. The sampled signal is typically measured to determine the power level, frequency, and/or signal shape (modulation) of the input signal. One typical directional coupler configuration is referred to as a hybrid coupler, a 3 dB coupler, a 3 dB hybrid coupler, a quadrature coupler, or a quadrature hybrid coupler, amongst other names. Regardless of how it is referred to, the quadrature hybrid coupler generally has the characteristics of dividing the input signal into two signals having equal powers and separated in phase by 90° when the four ports are properly terminated.
Typically, quadrature hybrid couplers are implemented by using two edge coupled transmission lines. However, planar circuit fabrication technologies, such as integrated circuit technologies, stripline technologies, and printed circuit board technologies, typically require a gap or spacing between the two coupled lines that is generally too small to be practically and reliably fabricated using conventional processes. Furthermore, the transmission lines typically used to provide proper coupling generally have large dimensions wide and long in order to achieve the impedance ranges typically used in radio frequency (RF) applications. One proposed solution has been the use of Lange couplers with two or more pairs of lines. However, even though Lange couplers alleviate the gap problem for planar processing technologies, the resulting size of Lange couplers is still problematic, as a quarter wavelength electrical length is still required for the coupler, resulting in couplers with a size typically exceeding that of any associated circuitry.